1. Field of the Invention
The present invention relates generally to fiber optic sensing systems and more particularly to processing data obtained from such systems.
2. Description of the Related Art
Fiber Bragg Grating (FBG) sensors, which are basically gratings along optical fibers that act as mirrors to light sources, provide many attractive characteristics compared to other sensing systems. These include their inherent light weight, small size, and immunity to electromagnetic radiation and radio-frequency interference.
These features enable using many sensors on a single optical fiber at either interval, arbitrary, or continuous spacing. Thus, a user can interrogate each sensor independently and obtain a distributed measurement over large structures. Because the gratings are multiplexed on a single fiber, many sensors can be accessed with a single connection to the optical source and detector. Hence, FBG sensors are highly attractive to the aerospace community. For example, U.S. Pat. No. 5,798,521 discloses an apparatus and method for measuring strain in structures using a plurality of FBG sensors and an improvement to this apparatus and method is disclosed in U.S. Pat. No. 6,566,648. A significant improvement in processing data obtained by the type of system described in these patents is described in U.S. Pat. No. 8,700,358. These patents are hereby incorporated by reference into this application.
As noted above, the current state of the art in fiber optic sensing systems and data processing is described in U.S. Pat. No. 8,700,358. In the system, hundreds of strain measurement locations (FBGs) exist along a single optical fiber. A laser light source excites the fiber, and reflections from each measurement location return to an optical detector. The reflections from each location are superimposed together and show up as one massive, conglomerate signal. The first main signal processing task is to demultiplex the conglomerate signal, i.e. to split out the individual signals from the huge conglomeration. Then the second task is to determine the measurement value for each individual measurement location. This second task consists of performing a centroid calculation and converting the result to strain. A summary of the overall process is given in FIG. 1.
The improvement in data processing described in U.S. Pat. No. 8,700,358 includes a much faster method of demultiplexing the conglomerate signal described above compared to that described in the U.S. Pat. No. 6,566,648 patent. Basically, the method involves dividing the raw data into a plurality of segments over time/wavelength. A Fast Fourier Transform is done on each of the segments in order to obtain frequency/positional data.
While this method significantly decreases the processing time and increases the refresh rate for an Optical Frequency Domain Reflectometry (OFDR) system, it does not provide efficient variable length segment resolution. Also, this method requires that the signal length be a power of two for maximum speed efficiency. In addition, this method requires processing data for the entire reference length of fiber selected (if a user wants to measure only data from a portion of the fiber, data for the entire fiber still needs to be processed).
Therefore, it is desired to provide an improved demultiplexing process for OFDR systems that provides efficient variable length segment resolution, does not require a signal length to be a power of two, maintains or decreases processing time and noise levels, allows a user to process data associated with only part of the fiber rather than the whole length, and adapts the resolution of a portion of the length of an optical fiber in real time.